1. Field of the Invention
The present invention concerns the field of hyperfrequency filters. Radiowave systems of large and average capacity often include hyperfrequency elimination band filters, the purpose of which is to eliminate fixed frequency disturbances. These filters can be realized by utilizing various technologies: wave guide, microstrip, coaxial line, etc.
For Radiowave systems, it is generally necessary that the filters are perfectly adapted in the pass band, i.e. that their return rate of the stationary wave (R.S.W.) is lower than 1.2, and at the same time that their overvoltage coefficient is high. Indeed, the insertion losses due to the filters must vary only slightly in the transmission band, particularly close to the rejected band. Furthermore, it is also preferable to be able to shift, in a substantial manner, the tuning frequency of the filter. Indeed, a standard filter could be designed for several channels of the hyperfrequency band, this same filter being able to be centered on one or another of these channels according to demand.
2. Description of the Prior Art
The hyperfrequency elimination band filters currently utilized do not allow one to obtain simultaneously these various characteristics, other than filters with cavities coupled to a wave guide. But these filters with cavities coupled to a waveguide have a complicated structure and relatively large dimensions.
The recent utilization of dielectric resonators placed in a coaxial structure has permitted the realization of elimination band filters having improved performance and requiring less space. But, for reasons that will be explained hereinbelow, the real tuning range of the resonators in a structure of this type is considerably reduced, lower than 10 MHz, due to the appearance of disturbing resonances in the transmitted band.